1. Field of the Invention
The present invention relates to nuclear power reactors, and more particularly, to a method and apparatus for sealing leaking welded penetrations to the head of a nuclear reactor vessel.
On typical pressurized water nuclear reactors, a plurality of control rod drive mechanisms (CRDM) are mounted on the vessel head. The mechanisms are located in pressure boundary housings and provide controlling motion to the neutron absorbing control rods. The components which make up the housings are assembled to each other and jointed with omega sealed welds. A canopy seal omega weld is between the reactor vessel head CRDM nozzle and the mating part (see FIG. 1). This weld has a tendency to develop cracks as a result of stress corrosion cracking. These cracks typically propagate through the weld until leakage occurs.
It is a common design to have reactor vessel head nozzles at every potential control rod position. A majority of the nozzles are used for control rods, a smaller number are used for core-exiting thermocouple instrumentation and several others (spares) are capped.
2. Description of the Prior Art
The repairs to leaking canopy seal welds are laborious and time consuming because they must be performed in a confined, highly radioactive area. Typically, the omega seal leaks have been fixed by shutting down the plant, draining down the reactor coolant to a level below the reactor vessel head and applying a weld overlay to the leaking location. This process takes a large amount of time, gives the workers a large dose of radioactivity due to the close proximity and contact with the reactor vessel head, and is no different than the existing weld, which means that it could leak again in the same location.